Plasma or gaseous discharge display and/or storage apparatus have certain desirable characteristics such as small size, a thin flat display package, relatively low power requirements and inherent memory capability which render them particularly suitable for display apparatus. One example of such known gaseous discharge devices is disclosed in U.S. Pat. No. 3,559,190, "Gaseous Display and Memory Apparatus", patented Jan. 26, 1971 by Donald L. Bitzer et al. and assigned to the University of Illinois. Such panels, designated a.c. gas panels, may include an inner glass layer of physically isolated cells or comprise an open panel configuration of electrically insulated but not physically isolated gas cells. In the open panel configuration which represents the preferred embodiment of the instant invention, a pair of glass plates having dielectrically coated conductor arrays formed thereon are sealed with the conductors in substantially orthogonal relationship. When appropriate drive signals are applied to selected pairs or groups of conductors, the signals are capacitively coupled to the gas through the dielectric. When these signals exceed the breakdown voltage of the gas, the gas discharges in the selected area, and the resulting charge particles, ions and electrons, are attracted to the wall having a potential opposite the polarity of the particle. This wall charge opposes the drive signal which produce and maintain the discharge, rapidly extinguishing the discharge and assisting the breakdown in the next alternation. Each discharge produces light emission from the selected cell or cells, and by operating at a relatively high frequency in the order of 30-40 kilocycles, a flicker-free display is provided. After initial breakdown, the wall charge condition is maintained in selected cells by application of a lower potential designated the sustain signal which, combined with the wall charge, causes the selected cells to be reignited and extinguished continuously at the applied frequency to maintain a continuous display.
The capacitance of the dielectric layer is determined by the thickness of the layer, the dielectric constant of the material and the geometry of the drive conductors. The dielectric material must be an insulator having sufficient dielectric strength to withstand the voltage produced by the wall charge and the externally applied potential. The dielectric should be a relatively good emitter of secondary electrons to assist in maintaining the discharge, be transparent or translucent on the display side to transmit the light generated by the discharge for display purposes, and be susceptible to fabrication without reacting with the conductor metallurgy. Finally, the coefficient of expansion of the dielectric should be compatible with that of the glass substrate on which the dielectric layer is formed.
One material possessing the above characteristics with respect to a soda-lime-silica substrate is lead-borosilicate solder glass, a glass containing in excess of 75 percent lead oxide. In an embodiment constructed in accordance with the teaching of the present invention, a dielectric comprising a layer of lead-borosilicate glass was employed as the insulator. However, chemical and physical reaction on the surface of the dielectric glass under discharge conditions produced degradation or decomposition of the lead oxide on the dielectric surface, thereby producing variations in the electrical characteristics of the gaseous display panel on a cell-by-cell basis. This degradation, resulting primarily from ion bombardment of the dielectric surface, caused the electrical parameters of the individual cells in the gaseous discharge device to vary as a function of the cell history such that over a period of time, the required firing voltage for individual cells fell outside the normal operating range, and the firing voltage varied on a cell-by-cell basis.
In order to avoid degradation of the dielectric surface resulting from ion bombardment in a gaseous discharge device, a refractory material having a high binding energy is utilized to protect the dielectric surface. A refractory material is one which resists ordinary treatment, is difficult to reduce and has a high binding energy such that its constituents remain constant even after prolonged use. It is also known in the art that the breakdown voltage in a gaseous discharge device may be lowered by utilizing a material having a high coefficient of secondary emission characteristics such as magnesium oxide. However, magnesium oxide reacts with the dielectric glass during fabrication and has a tendency to crack or craze during the fabrication process. In addition, the secondary emission characteristic of magnesium oxide may be too high for certain applications.
With respect to gas panel fabrication and test, the conventional process requires a significant burn-in time in the general order of 16 hours as the final step. When alternate line testing was employed in a panel having a magnesium oxide dielectric surface, a lowering of the memory margin, the difference between the maximum and minimum sustain voltage, was noted in the tested lines as compared to the non-tested lines. This phenomenon, known as alternate line aging, reduced the memory margin of the tested cells below acceptable limits resulting in rejection of a substantial number of panels.